This invention relates to surface acoustic wave resonator devices having grating reflectors and more particularly to resonator devices used as bandpass filters and oscillator frequency control elements in the VHF or UHF frequency ranges.
There is frequently a requirement in communications and other electronic fields for filters which process a very narrow frequency range (i.e., bandpass filters or oscillator frequency control elements). Depending on the specific requirements and the frequency range under consideration, there are available various approaches in fulfilling this requirement. One approach frequently used is a crystal resonator employing a quartz crystal. It is extremely difficult, however, to construct such a filter for use in the VHF and UHF bands of the resonator because resonance frequency must be determined by the thickness of the quartz crystal.
Surface acoustic wave resonator devices having a pair of grating reflectors are generally employed as filters in such high frequency ranges. This kind of surface acoustic wave resonator devices are described, for example, in U.S. Pat. No. 3,886,504 and U.S. Pat. No. 4,166,258. In such resonators, the grating reflectors comprise a plurality of parallel conductors. The generated waves propagate in a direction perpendicular to the longitudinal orientation of the conductors but also in other directions which are not aligned with the longitudinal orientation of the conductors. Therefore, a pair of reflectors confines not only the fundamental mode but also higher transverse modes. Such higher transverse modes cause spurious responses in the attenuation characteristics of the surface acoustic wave resonator.
One method for eliminating much higher transverse modes is by apodizing input and output electrodes. This is reported by W. H. Haydl et al. in a paper entitled "Multimode Saw Resonators--a Method to Study the Optimum Resonator Design," presented at the 1976 IEEE Ultrasonics Symposium Proceeding, pages 287-291 (see FIG. 20), 1976. With such a structure, however, the effective finger number of the input and output electrodes is necessarily reduced; as a result, the equivalent coupling coefficient of the device is lowered. Therefore, this method has a disadvantage of bandwidth reduction.
Another method for suppressing the higher transverse modes requires utilizing a plurality of parallel interconnected conductors extending through a wave-guide wherein the position of the conductors outside the waveguide are narrower than the portion within the waveguide. A modification of this method utilizes a plurality of parallel conductors extending through a waveguide wherein the portion of the conductors outside the waveguide are isolated (strips or lines of dot electrodes) whereas the conductors within the waveguide are solid interconnected conductors. These methods are described by Hiroshi Shimizu et al in a paper entitled "Control of Waveguide Characteristics and Suppression of Spurious Transverse Modes for SAW Resonators" (see FIG. 3 (a), (b), p. 80) presented at the Japanese Acoustic Society, June 1979. With these methods, however, appropriate electrodes are formed external to a waveguide. Therefore, in utilizing such methods, the substrate must necessarily be wider than the effective length of the input and output transducers to accommodate the external conductors. This produces the disadvantage of requiring a large resonator device.
U.S. Pat. No. 3,952,269, also discloses a surface acoustic wave device which utilizes isolated parallel conductors. The device, however, is used as a delay line which exhibits accurate time delay and insertion phase. This patent neither discloses, or suggests the use of grating reflectors to suppress higher transverse modes.